Displacement sensors capable of measuring over a large range (3-13cm) available at present are typically of the mechanical and/or electrical, i.e. variable resistance or induction types. Devices of this type are usually less accurate and variable resistance or induction type devices tend to lack long term stability.
The vibrating wire measuring principle provides a very reliable method of measuring small deformations with high precision. Unidirectional vibrating wire strain monitors, in general, comprise a thin wire anchored at its ends to produce a nominal tension therein. The ends are movable relative to one another, for example in response to deformation, so as to produce a change in the tension of the wire. Autoresonance or plucking circuits are typically used to force electromagnetically the wire to vibrate and to determine subsequently the resultant resonant frequency of the wire. Since the resonant frequency of vibration is dependent, among other things, on the tension in the wire, a change in distance between the ends of the wire will result in a corresponding change in the resonant frequency of the wire. Unfortunately, the range of measurement of such vibrating wire strain monitors is limited because the deformations must remain within the elastic range of stretch of the wire.
In U.S. Pat. No. 5,038,622, issued Aug. 31, 1991, a calibrated extension spring is secured to the end of the vibrating wire to achieve an increase in range. It has been found, however, that difficulties are encountered in obtaining a resonant frequency for the wire if an extension spring is used that possesses relatively low stiffness, with respect to the wire, in the lateral and longitudinal direction with the vibrating wire only lightly stressed.